if everything else is running (turn table, clocks counting down , and the fans are running) then most likely the magnetron is bad, these are expensive and dangerous to replace, the microwave can hold lethal amounts of voltage even when unplugged, this repair is best left to a tech. please have a service tech work on this,
The U. S. Consumer Product Safety Commission (CPSC) warns against do-it-yourself repairs of microwave ovens. Each year, on average, four people die from electrocution trying to repair their microwave ovens. Microwave ovens use high voltage which makes it particularly hazardous for consumers to remove the cover and touch electrical parts. The possibility for electrical shock from a microwave oven still exists even after the oven is disconnected from the power source. Should certain internal devices fail, even unplugged ovens can cause serious injury. The Commission strongly recommends that consumers have professionals repair microwave ovens because electrical appliances can be hazardous if consumers expose live electrical parts.
/!\ WARNING ELECTROCUTION HAZARD HIGH VOLTAGE Removing the cover of your microwave oven and touching an electrical part can kill you. Have a professional repair your microwave oven